Laser radiation of visible and/or infrared wavelengths is used for a variety of purposes in modern armed forces. For example, pulsed or intensity modulated laser radiation is used in fire control systems to designate the target, in missiles to guide the missile to a chosen target, in aircraft for providing control signals to the control surfaces, and in many other systems.
The use of lasers to designate targets is the particular concern of this invention. It is clearly in the interest of the operator of a target so designated, be it a vehicle, ship, or aircraft, to be alerted to the presence of laser radiation and the direction from whence it comes. This detection and localization of the threat must occur at the target and must detect the threat amidst a pattern of disturbing background radiation. The wave length, pulse length and pulse repetition frequency should also be determined to learn the nature of the threat. A knowledge of all these factors will enable the target to react properly, by engaging the laser source, maneuvering to avoid the beam, using smoke or other types of false target generators to mask itself, etc.
Different targets such as ships, vehicles, helicopters, airplanes and satellites make different demands on a detector system in both the width of the solid angle to be scanned and the angle resolution required in the azimuth and elevation directions. For example, for vehicles in the field such as tanks and trucks, the threat, typically other tanks or low-flying helicopters, comes from a source in elevation angular ranges near the horizontal plane. The required detection range is thus 360.degree. in the azimuth plane and an elevation plane angle of .+-.30.degree.. By elevation plane the applicant intends all planes perpendicular to the horizontal or azimuth plane.
The nature of vehicular travel is such that large changes in azimuth usually occur slowly while elevation changes can occur much more rapidly over the same tim period. However, these elevation changes are usually of a small order, typically a few feet. Therefore the detection system must provide precise azimuth angle measurements of a laser radiation source with respect to the system of coordinates of the vehicle. Elevation angle measurement is of much less importance. The full sensitivity of the radiation detection over the entire demanded elevation range must be insured, however, in order to locate the radiation source precisely.
With other targets, particularly helicopters, airplanes, and satellites, their high mobility requires precise angular resolution in both the azimuth and elevation direction. The solid angle ranges should be 360.degree. in the azimuth plane and +90.degree. in the elevation plane.
In addition to the different requirements for the angular detection ranges of the sensors, the user also needs to be able to detect radiation from lasers of widely different pulse rates, wavelength and frequency.
The great number of different military radiation sources range from short-wave visible light up to thermal infrared. The wavelength ranges for frequency displaced Excimer lasers are 0.4 .mu.m, for laser diodes 0.85 to 0.95 .mu.m, for alexandrite lasers 0.75 to 0.85 .mu.m, for Nd:YAG 1.06 .mu.m to 1.32 .mu.m, for Nd:YAG-Methane Raman Shifted 1.52 .mu.m, for Erbium 1.65 .mu.m, for Holmium 2.12 .mu.m, for Deuterium Flouride 3.6 .mu.m to 4.0 .mu.m, and for CO.sub.2 lasers 9.5 .mu.m to 11.5 .mu.m. The radiation detector must have a broad band spectral sensitivity to receive and detect all these different laser sources. Additionally, filters cannot be used to reduce interference from background lighting as such filters also reduce the spectral sensitivity of opto-electronic detectors.
The laser detector must be able to operate faultlessly despite the mobility of the target objects. If sunlight impinges directly upon the detector the rate of false alarms should still remain reasonably low. Additionally the laser detector must be insensitive to other artificial lighting sources such as stroboscopes, flashlamps, canon fire, flares, or any other type of light flash.
The detector device must furthermore have a large dynamic signal range so that operation is not impaired by different weather conditions, varying distances between the warning sensor and the laser source, changing pulse energies of the laser, changing incident locations of the laser beam with the target, and the turbulent fluctuations of the atmosphere over the radiation cross section. Disturbances of the angle measurements through secondary reflections of the laser radiation in the environment and at the target must also be prevented.
Laser radiation detector devices are known from DEP No. 33 23 828 as well as DE-OS No. 35 25 518. These devices contain at least a first and second detector stage, each stage having an opto-electric transducer with associated discrete optics, the stages acting together to detect laser radiation over a preset solid angle. A certain amount of overlap occurs between each of these preset solid angles to insure complete coverage. The discrete optics are arranged in azimuth planes with a first and second wave guide for each of the discrete optics. The wave guides lead respectively to the first and second detector stages with all wave guides that lead to the first stages having the same length. The wave guides that lead to the second detector stage are of different lengths, their length being graded incrementally in the direction of increasing azimuth angle. The incremental increase in length causes an incremental increase in transit times. A transit time measuring device is coupled to each respective first and second detector stage to determine the beginning and end of the transit time and, as a function of it, to determine the azimuth angle of the incident laser radiation.
The previously described devices use a pair of fiber optic wave guides behind a common discrete optics. Depending on the magnitude of the solid angle to be scanned, numerous such sets of discrete optics are necessary. Adjacent discrete optics have overlapping discrete solid angle ranges. All first wave guides for the discrete optics are led to a first detector, which upon arrival of a laser impulse triggers a transit time measuring circuit. All second wave guides, which are of different lengths, are connected with a second detector, which, when activated stops the transit time measuring circuit. The time difference between the starting and the stopping of the counter circuit, which is a function of the length of the second wave guide, is a measure of which solid angle in the azimuth plane the laser radiation is coming from. As there is some overlap between each of the pairs of detector stages and their respective solid angle detection zones, all around detection is ensured.
Although ideally the laser radiation would impinge on the optics of one set of discrete optics only, in actuality the radiation also impinges on the adjacent sets of discrete optics. The consequence of this is that the transit time measuring circuits of the adjacent detector stages provide a timing signal, one of them preceding the main detector pulse and one of them succeeding the main pulse. In order to determine the precise azimuth solid angle of the incident radiation, an interpolation operation is carried out to determine the temporal center of these three received pulses. The calculations necessary to do so are described in DE-OS No. 35 25 518.